This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-141784, filed May 15, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a circuit for driving a heater at a constant power, which is incorporated in, for example, a limiting-current type oxygen sensor.
2. Description of the Related Art
FIG. 4 shows a limiting-current type oxygen sensor 1. The oxygen sensor 1 comprises an ion conductor 2, two electrodes 3a and 3b, and a heater 4. The ion conductor 2 is made of, for example, stabilized zirconia. The electrodes 3a and 3b are made of porous material and provided on the upper and lower surfaces of the ion conductor 2, respectively. Gas containing oxygen ions is supplied to the electrode 3a, diffused and flowing at a regulated speed. As the oxygen ions move in the ion conductor 2, a current is generated. The current depends on the voltage applied across the ion conductor 2 and the temperature of the ion conductor 2.
Therefore, in the limiting-current type oxygen sensor 1, the heater 4 is arranged above an oxygen-detecting element (not shown). The heater 4 heats the oxygen-detecting element to a monitoring temperature of about 400xc2x0 C. while the element is performing its function. The heater 4 is a pattern of platinum paste, which has been printed and sintered.
To keep heating the oxygen-detecting element at 400xc2x0 C., the heater 4 needs to be driven at a constant power of, for example, about 1.5W. Formed by printing technology, heaters of this type 4 differ in thickness, width and the like. Inevitably, they have greatly different resistances. Consequently, the voltage applied to supply a constant electric power to one heater differs from the voltage applied to supply the same electric power to another heater.
Hitherto, a drive circuit of the structure shown in FIG. 3 has been used to drive a heater 4 of the type illustrated in FIG. 4. The drive circuit comprises a transistor 31, a diode 32, a resistors 33, 37 and 38, a trimmer potentiometer 34, a resistor 35, and an operational amplifier 36. The transistor 31 drives the heater 4, which is secured to the detecting element 2 of a limiting-current type oxygen sensor 1. The emitter of the transistor 31 is connected to the diode 32, which in turn is connected to a power supply. A voltage is applied to the heater 4 via the collector of the transistor 31. The collector of the transistor 31 is connected to the ground by the resistor 33, trimmer potentiometer 34 and resistor 35. The dial 34 adjusts the voltage, which is applied to one input terminal of the operational amplifier 36. The base of the transistor 31, which is biased by the resistor 37, receives the output of the operational amplifier 36 through the resistor 38. It is therefore necessary for the user to turn the dial of the trimmer potentiometer 34 so that an appropriate voltage may be applied to the heater 4.
For the manufacturer of the sensor 1 it is necessary to determine the voltage that should be applied to the heater 4 so that heater 4 may be driven at a predetermined power (e.g., 1.5W) and to record the data representing the voltage in a recording medium such as a data sheet and to secure the medium to or in the sensor 1. For the user of the sensor 1 it is necessary to turn the dial of the trimmer potentiometer 34 very carefully so that the voltage represented by the data recorded in the medium may be applied to the heater 4. Further, once the dial 34 has been turned so, the user needs to lock the rotor of the dial 34 by means of, for exampling, paint locking.
The present invention has been made in view of the foregoing. An object of the invention is to provide a circuit that is designed to drive the heater at a constant power in a sensor and in which the voltage to apply to the heater need not be adjusted at all, rendering it unnecessary for the manufacturer or user of the sensor to take measures to adjust that voltage.
According to one aspect of the invention there is provided a circuit for driving a heater at a constant power. The circuit comprises: a current-limiting resistor to be connected in series to the heater provided in a limiting-current type oxygen sensor; and a drive element for driving, at a constant voltage, a series circuit constituted by the current-limiting resistor and the heater.
The power consumed in the heater can remain constant even if the heater has a resistance different from the design value. It is therefore unnecessary to secure a recording medium, such as a data sheet, to the medium to or in the limiting-current type oxygen sensor. Nor is it necessary to adjust the voltage applied to the heater. Thus, there will not arise a mismatching between the sensor and the data representing the voltage that should be applied to the heater so that heater 4 may be driven at a predetermined power. In addition, the user of the sensor need not turn any dial to apply an appropriate voltage to the heater. The circuit according to the invention can supply a constant power to the heater for a long period of time.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.